1. Field of the Invention
The present invention relates to a sheet-fed rotary printing machine.
2. Description of the Related Art
In general, in printing of the type employing such a sheet-fed rotary printing machine, the position of printing paper on the outer circumference of an impression cylinder is significantly important in order to achieve accurate printing without involving any dislocated printing or doubled printing. Therefore, before the printing paper is fed to the bottom of the blanket cylinder, it is necessary to feed the printing paper to a predetermined position on the outer circumference of the impression cylinder.
In order to meet the above-described requirement, the following type of printing machine is widely employed.
As shown in FIG. 20, the illustrated printing machine generally includes an impression cylinder 61, a paper cassette 62 disposed in face-to-face relationship therewith, and an endless belt 63 disposed between the components 61 and 62 for feeding printing paper P. Sheets of printing paper P are stacked in the paper cassette 62, and a suction member 64 is disposed for vertical movement above the paper cassette 62. Each sheet of the printing paper P is held by the suction member 64 by suction to carry it to the feed belt 63. When the printing paper P is transported to one end of the feed belt 63, the printing paper P is pressed into contact with the belt 63 by means of a feed roller 65, and the suction through the suction member 64 is cancelled to feed the printing paper P toward the impression cylinder 61. In addition, the printing paper P is fed to a paper feed member 66, and each sheet of the printing paper P is properly registered by position limiting plates 67 disposed upright on opposite sides of of the paper feed member 66, thus being fed to a predetermined position on the outer circumference of the impression cylinder 61. Thereafter, the printing paper P is held against the outer circumference of the impression cylinder 61 by a retention pawl 68. Then, as the impression cylinder 61 rotates, the printing paper P moves to a predetermined position on the impression cylinder 61 which is maintained in contact with the bottom of the blanket cylinder 69 so that printing is effected.
The above-described related art printing machine, however, involves the following problems.
The suction member 64, the feed belt 63, and the paper feed member 66 constitute in combination means for feeding the printing paper from the paper cassette to the outer circumference of the impression cylinder. Accordingly, the number of parts needed increases and hence the size of the printing machine increases. This may result in an increase in production cost.
The printing paper P is supplied from the suction member 64 to the feed belt 63 with the printing paper P being pressed in contact with the feed belt 63 by the feed roller 65. This pressure may cause damage to a surface of the printing paper P.
FIG. 23 shows another example of this conventional type of sheet-fed rotary printing machine. In this example, a recess 92 is formed in the outer circumference of an impression cylinder 91, and a support shaft 93 having a plurality of gripping pawls 94 (only one of which is shown) is disposed in the recess 92, with the support shaft 93 being capable of rotating to-and-fro. The arrangement of such a printing machine is such that the leading edge of the printing paper P supplied from a paper supply table 95 is gripped by each of the gripping pawls 94. As the impression cylinder 91 rotates, the printing paper P is subject to printing by means of a blanket 96a laid over a blanket cylinder 96.
In such an arrangement, a margin of the printing paper P which is adjacent to its leading edge is employed as a margin which is directly gripped by the gripping pawls 94. It is therefore impossible to effect printing onto this margin. Accordingly, there is a problem in that it becomes impossible to effect printing over the entire surface of the printing paper P.
In order to solve the above-described problems, the present applicant proposed the following printing machine in Japanese Patent Laid-open No. 132348/1986.
As shown in FIG. 21, this printing machine includes an impression cylinder 81 having an outer circumferential surface through which a plurality of air passage holes 82 are formed, and the air passage holes 82 communicate with an air passage 83 formed within the impression cylinder 81. Opposite ends of the impression cylinder 81 each have a flange valve 88 which can rotate with respect to the impression cylinder 81. One of the flange valves 88 has an suction hole 89 which is formed therethrough while the other has a supply hole 90 which is formed therethrough. The suction hole 89 and the supply hole 90 are respectively connected to pipes 84 which extend from a suction device and an air supply device. As shown in FIG. 22, as the impression cylinder 81 rotates, the air passage 83 is adapted to provide communication between the air supply device and the suction device.
In this arrangement, during printing, the air passage 83 is made to communicate with the suction device to produce a small negative pressure through the air passage holes 82, thereby causing the printing paper P in a paper cassette 86 to be drawn against the impression cylinder 81 by suction through the air passage holes 82. This suction prevents the dislocation of the printing paper P. After completion of printing, the air passage 83 is made to communicate with the air supply device to produce a positive pressure through the air passage holes 82, thereby causing the printing paper P drawn by suction through the air passage holes 82 to be released from the impression cylinder 81.
In the above-described printing machine, the printing paper P is drawn against the outer circumferential surface of the impression cylinder 81 by suction through the air passage holes 82. Therefore, as the impression cylinder 81 rotates, the printing paper P is positively fed along the outer circumference of the impression cylinder 81. However, if the printing machine is used continuously or at high speed, the printing paper P may be drawn against the impression cylinder 1 with the leading edge of the printing paper P projecting forward from the air passage holes 82. In this case, it becomes impossible to effect printing on the projecting leading end portion. This continues to be a problem in that it is impossible to effect printing over the entire surface of the printing paper P.